1. Field of Invention
The present invention relates to a gamma correction circuit and a video display apparatus that gamma corrects a first polarity video signal and a second polarity video signal.
2. Description of Related Art
In a video display apparatus incorporating a liquid-crystal panel that includes a data driver IC and a scanning driver IC each constructed of thin-film transistors (TFTs) on a glass substrate, the operating speed of the data-line driving circuit and the scanning-line driving circuit is substantially lower than that of circuitry formed on a silicon substrate primarily because of the response of a liquid crystal crystalline of silicone. Therefore, it is required to match the frequency of a video signal to the operating speed on a sampling side.
It is contemplated that the pixel signal contained as serial data in the video signal is expanded in phase and video presentation is performed using the phase-expanded video signal. As shown in FIGS. 19A and 19B, the data processing circuit block 10 in the video display apparatus is connected to a phase expansion circuit 30 that expands an input video signal VIDEO to six phases and in response to a control signal from a timing circuit block 20, outputs panel driving video signals V(i) (i=1-6) from respective output terminals OUT 1-OUT 6 on a per phase basis. The panel driving video signals V(i) are respectively applied to data signal lines 112 by supplying lines 132 connected to sampling switches 134 in a liquid-crystal panel 110 that are connected to respective six pixels that are arranged in a horizontal direction. Since the panel driving video signals V(i) are those into which the input video signal VIDEO is six-phase-expanded by the phase expansion circuit 30, each panel driving video signal V(i) contains a pixel signal for every six pixels, and the frequency of the panel driving video signal V(i) becomes lower than that of the input video signal. VIDEO. For this reason, even if the operating speed for a data driver IC 130 and a scanning driver IC 120 each constructed of thin-film transistors is slow, the data driver IC 120 130 samples a pixel signal PD for each data signal line 112 at the sampling switches 134 out of the panel driving video signals V(1)-V(6) fed to terminals VIN1-VIN6, respectively, based on the sampling signal output from a shift register 136 for driving the sampling switches 134.
In the video display apparatus, an image of high image quality is presented if one-dot polarity reversal displaying is performed.
Conventionally, as shown in FIG. 20, a polarity reversal circuit 43 is arranged as a front stage prior to the phase expansion circuit 30. A signal output circuit 42 in the polarity reversal circuit 40 produces two types of video signals in reverse polarity from the input video signal VIDEO. Selectors 44a, 44b constructed of analog switches switch the polarity of the video signal applied to each sample/hold circuit within the phase expansion circuit 30. A gamma correction circuit (not shown) performs analog gamma correction and is conventionally arranged as a front stage prior to the polarity reversal circuit 40.
The conventional video display apparatus needs a large component count because it essentially requires that a signal is subjected to analog gamma correction prior to polarity reversal process, then separated through the polarity reversal circuit 40 into a positive polarity signal and a negative polarity signal, and then selected in a time-division manner by the analog switches.